1. Field of the Invention
The present invention relates to motor vehicle brake systems and more particularly to a corrosion resistant cast-in insert ABS exciter ring integrated with either a disk rotor or, for drum brake systems, a wheel hub.
2. Description of the Problem
Brake units for motor vehicles should provide smooth braking with reasonable service life. This need has been met with disk brake rotors made from relatively inexpensive gray iron castings. Gray iron is however highly susceptible to corrosive attack, particularly in the operating environment of vehicles where brake components are open to the air, subject to substantial transient heating and exposed to water and salt water spray. In regular use, the working surfaces of the disks are rubbed clean by contact with the disk pads, which are typically made of a composite material and which rub off corroded areas. However other areas of the brake disks are not swept by the brake pads and thereby cleaned of corrosion. Prior to anti-lock braking systems, such concerns were not paramount with brakes which were frequently in use, since the rotor is a regularly replaced part and the remaining areas subject to attack were non-critical.
With the advent of anti-lock braking systems other sections of the disk brake rotor can take on importance. Among other sections of a typical disk rotor of mechanical importance is an anti-lock brake system exciter ring which has typically been cast as one piece with the rotor. The exciter ring is a cylindrical section of the rotor having a common axis of rotation with the rotor. A plurality of teeth are positioned in a ring, flat in the plane of rotation of the rotor and outwardly oriented on the exterior of the ring to pass closely by a stationary sensor. The stationary sensor is a variable reluctance sensor which generates an electrical pulse train as a function of the varying magnetic flux leakage between the sensor head and the exciter ring. The frequency of the resulting electrical pulse train indicates the rotational speed of the wheel on which the rotor is mounted. The generation of clean pulse train is greatly aided by having teeth of uniform shape, size and spacing. Corrosion can greatly compromise all of these factors, resulting in difficulty in detecting the passage of teeth and gaps and resulting in a corrupted pulse train.
It is known that coating parts suppresses corrosion. Corrosion protection coatings can be used such as that described in U.S. Pat. No. 5,569,543 and those supplied by Magni Corp., including the Magni 109 and Magni 111 coatings. Such coatings can be easily compromised when applied to exciter rings since the sensor usually needs to pass within very close proximity to the teeth and, consequently, the chance exists for removal of the coating from the teeth, which again leaves the teeth exposed to corrosive agents. In addition, such coatings are relatively expensive and their long term durability under all of the widely varying conditions of vehicle use is not well known.
It is known to make disk rotors out of more than one material, although no application of such an approach to solving the problems of ABS exciter rings is known to the inventor. German Laid Open Application 42 37 655 describes a brake disk for a motor vehicle disk brake system. The rotor comprises two abrasion rings, including an inner ring made of iron and an outer ring made of a composite fiber material. The rings are bonded to one another, preferably using rivets. The application mentions the possibility of casting the iron disk onto the composite fiber disk. The application is not directed primarily to corrosion problems but rather to improving weight balance, reducing the potential for cracking and reducing brake vibration.
The problem of corrosion of exciter rings for anti-lock brake systems is not limited to disk brake systems, but is also an issue with drum brakes. In drum brakes the exciter ring has not been an integral part of any part of the working brake, but rather has been a separate part press fitted on the end of a wheel hub. Such hubs are frequently made from nodular iron, which like gray iron is highly susceptible to corrosive attack. Improper control of the fitting process can result in exciter rings becoming displaced from an optimal position.
According to the invention there is provided a rotational element for a wheel comprising a cast iron segment and an anti-lock brake system exciter ring segment, which adheres to the cast iron segment as a cast-in insert. The anti-lock brake system exciter ring segment is made of a corrosion resistant, magnetic material having a higher melting point than the cast iron segment. The exciter ring includes at least one tang penetrating the cast iron segment. The exciter ring is preferably made from ferritic stainless steel where magnetic anti-lock brake sensors are to be used. For disk brake systems the rotational element serves as a disk brake rotor. In vehicles using drum brakes the rotational element serves as a wheel hub.
Additional effects, features and advantages will be apparent in the written description that follows.